drm/amdgpu: potential NULL dereference in debugfs code

debugfs_create_file() returns NULL on error, it only returns error
pointers if debugfs isn't enabled in the config and we checked for that
earlier so it can't happen.

Fixes: 4f4824b5

 ('drm/amd/amdgpu: Convert ring debugfs entries to binary')
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>